NEW YORK — Harper Lee’s attorney has provided her fullest accounting yet of how she came upon the manuscript for “Go Set a Watchman,” which comes out Tuesday.

She is also raising the possibility of a third book by the author of “To Kill a Mockingbird,” a suggestion challenged by a leading biographer.

In an op-ed piece posted late Sunday on the Wall Street Journal’s website, Tonja Carter wrote that last summer she was at a gathering of Lee’s friends and family members when “talk turned” to a possible second novel by the author of “To Kill a Mockingbird.” According to Carter, it was the first time she had heard of the book’s existence. She then searched in a safe-deposit box that contained some of Lee’s papers and found “Watchman” underneath “a significant number of pages of another typed text.”

“I immediately went to Nelle,” Carter wrote, referring to Lee’s first name, which she is often called by. “I said, “‘Nelle, when I was in the safe-deposit box, I found something.’ She said, ‘What?’ I said, ‘It’s a manuscript of a novel called ‘Go Set the Watchman.’ She said, “It’s ‘Go Set a Watchman.’ ”

Calling herself Lee’s “estate trustee, lawyer and friend,” Carter also disputed recent reports that she had learned of “Watchman” a few years earlier, during a review of some of Lee’s papers that included a Sotheby’s appraiser and then-Lee attorney Samuel Pinkus. Contradicting statements by Pinkus, Carter said she was not in the room at the time “Watchman” might have been discussed and that no one mentioned the book to her.

Lee sued Pinkus in 2013, alleging that he had “duped” her into signing over the copyright of “Mockingbird.” They settled months later.

Until Sunday, Carter had said little about “Watchman.” Set in Lee’s fictional Maycomb, Alabama, in the 1950s, “Watchman” was written before “Mockingbird,” but takes place 20 years later. Lukewarm reviews and reports of the book’s raw account of an elderly, racist Atticus Finch have not prevented “Watchman” from holding the No. 1 spot on Amazon.com’s best-seller list. “Mockingbird,” published in 1960, was No. 2 as of midday Monday.

Carter has been the object of skepticism since February, when HarperCollins revealed the stunning news that a second novel from Lee would be released. Many wondered whether the 89-year-old Lee, who has poor hearing and eyesight and lives in an assisted facility in her native Monroeville, had approved the publication. Alabama state officials, responding to at least one complaint, met with her and concluded she was alert and aware of the book’s release.

At the end of her op-ed piece, Carter suggested that the “typed text” in the box of papers could be an early “Watchman” draft or a third book “bridging” the stories of “Mockingbird” and “Watchman.” Lee’s literary agent, Andrew Nurnberg, has previously said that he has seen old letters between Lee and her then-agent that indicated “Mockingbird” was part of a planned trilogy.

“In the coming months, experts, at Nelle’s direction, will be invited to examine and authenticate all the documents in the safe-deposit box,” Carter wrote.

Charles J. Shields, author of “Mockingbird: A Portrait of Harper Lee,” said Monday that he had never seen any indication that Lee was working on a trilogy.

“To the contrary, her agent, Annie Laurie Williams, wrote to her a year after the publication of ‘To Kill a Mockingbird,’ encouraging her to have another novel soon,” Shields wrote in an email.

“‘Go Set a Watchman’ was complete at that time, but unpublished. If a trilogy was planned, why didn’t Lee, her editor, and her agent turn their attention to that manuscript?”

Shields added: “A shaggy-dog story seems to be developing about lost manuscripts.”